For a mobile communication system of CDMA mode, because of the coexistence of multiuser in the same channel and the complexity of the wireless environment, when a receiver receives an expected signal, it is inevitable that the signal is influenced by Fading, Multi-Access Interference (MAI) and Inter Symbol Interference (ISI). In the Multi-User Detection (MUD) method, the interference signal can be used effectively and interference may be eliminated or weakened via de-spread and multiuser joint channel equalization, so that the system performance may be improved greatly.
Before a multiuser detection is performed, the receiver should preknow the channelization code of the user accessed in the current time slot. Usually, the channelization code is obtained in the following way: the receiver predetects the channel estimation window that is activated, then finds the corresponding channelization code using the relation between the channel estimation window and the channelization code that is established during the code channel allocation process, and performs the multiuser detection using the code channel in which the channelization code exists.
In the existing Time Division Duplex Mode of CDMA (TDD-CDMA) communication system, there are three methods for code channel allocation: specific assigned allocation mode (Specific), default allocation mode (Default) and common allocation mode (Common). In the Specific mode, the relation between the channelization code and the channel estimation window is informed by an upper layer, a plurality of channelization codes that belong to the same user occupy the same channel estimation window, and the window position is designated randomly. In the Default mode and the Common mode, the relation between the channelization code and the channel estimation window is defined according to a default rule. Because the allocation of the Specific mode is too flexible and extra signaling information is required, it is usually not adopted in the practical system development. Here, the method for code channel allocation will be illustrated with respect to the Default mode and the Common mode.
In the Default allocation method, a plurality of channelization codes of the same user occupy one or more channel estimation windows, and the position of the estimation window is related to the channelization code. In the Common allocation method, a plurality of channelization codes of all the users occupy the same channel estimation window, and the position of the channel estimation window is related to the channelization code number. The Default allocation method may be applied to an uplink or a downlink, while the Common allocation method can only be applied to a downlink.
The code channel allocation in the Default mode and the code channel allocation in the Common mode will now be illustrated by taking the case in which the maximum number of channel estimation windows is 8 (i.e., K=8) as an example.
Referring to FIG. 1, it is a schematic diagram of the code channel allocation in the Default mode when K=8. Wherein, m(i) is the ith channel estimation window, cj(i) is the ith channelization code with a spread factor of j (1<=i<=j). It can be seen from FIG. 1(a) that the same channel estimation window position corresponds to a plurality of channelization codes, and the plurality of channelization codes may have the same spread factor (parallel walsh branch), or they may have different spread factors (home walsh branch). For example, the first channel estimation window (m(1)) corresponds to c16(1), c16(2), c8(1), c4(1), c2(1) and c1(1) at the same time, where c16(1) and c16(2) are both parallel walsh branches whose spread factor is 16; c16(1), c8(1), c4(1), c2(1) and c1(1) are home walsh branches whose spread factors are 16, 8, 4, 2 and 1 respectively; c16(1) and c16(2) belong to c8(1), c8(1) belongs to c4(1), c4(1) belongs to c2(1), and c2(1) belongs to c1(1).
Referring to Table 1, it is a schematic table showing the code channel allocation in the Common mode when K=8.
TABLE 1m1m2m3m4m5m6m7m8100000001 or 9 codes010000002 or 10 codes001000003 or 11 codes000100004 or 12 codes000010005 or 13 codes000001006 or 14 codes000000107 or 15 codes000000018 or 16 codes
It can be seen from Table 1 that a channel estimation window position corresponds to a plurality of channelization code numbers. For example, the first channel estimation window (m(1)) corresponds to 1 or 9 channelization codes at the same time.
In the existing communication process, the receiver of a base station is preinformed of the code channel allocation information of the current accessed user, and the receiver of a terminal also knows its own code channel allocation information. In the Default mode, if the practical transmitted code channel is the code channel allocated, the receiver performs a multiuser detection directly with the allocation information that is known. However, during the practical transmission process, on one hand, because the traffic varies from high to low, part of the code channels allocated are not involved in transmission; on the other hand, due to the complexity of transmitting beamforming and wireless environment, part of the transmission code channels suffer from severe attenuation and fading, thus a situation may be caused in which the code channels that actually need to participate in detection on the receiving end are different from the code channels allocated. In such a case, the receiver cannot know exactly the code channels that are involved in transmission and the fading status. Thereby, difficulty may be brought about for the multiuser detection: computational complexity should be increased in order to find the code channels that are involved in transmission practically but are not weakened by fading (i.e., the code channels that need to participate in the multiuser detection).
In the Common mode, the receiver cannot know the exact number of the transmitted code channel with the predetected channel estimation window that is activated. For example, in FIG. 1(b), supposing that the transmitting end occupies 1 channel estimation window for information transmission, and the receiver performs channel estimation using a training sequence and obtains the channel estimation window m(1) that is activated. In the Common mode, although the first channel estimation window (m(1)) corresponds to 1 or 9 channelization codes at the same time in the code channel allocation, the receiver usually performs multiuser detection with the maximum channelization code number so as to avoid the situation that a user may be missed out. In other words, the receiver performs multiuser detection with 9 code channels. Thereby, 8 interfering code channels are introduced, so that the complexity of multiuser detection is increased greatly and the performance of multiuser detection is lowered.